The present invention relates to a device for measuring current by using an optic fiber. The measuring principle is based on the FARADAY effect in a monomode optic fiber.
A device has been proposed device for measuring current in a conductor by using the FARADAY effect in a slightly birefringent monomode optic fiber. The fiber is wound, in a certain number N of turns, around the conductor.
The light, polarized linearly and preferably monochromatic, is subjected, due to the effect of the magnetic field parallel to its propagation direction, to a polarization rotation called "FARADAY rotation" of angle F, which is given by the following relation: EQU F=VNI
where V is the VERDET's constant of the optic medium, which, in the case of a silica optic fiber equal, is to 3 microradians/A for a wavelength of 780 nm.
Measurement, or detection, of this polarization rotation angle F consequently enables the value of the current intensity I in the conductor to be deduced.
The article by Messrs. ROYER, KEVORKIAN, RIVAL, TURC and CARDOT entitled "Optic fiber current sensor", published in the "Revue Generale de l'Dlectricite", 1990, n.degree.4, pages 62 to 67, describes two possible embodiments of a current sensor operating according to this principle, and differentiated by their principle for detecting, (i.e., for measuring) the polarization rotation angle F.
In both cases, light emission in the monomode optic fiber is achieved by using a laser diode which constitutes the required monochromatic light source.
The first configuration, described in this article, uses a detection called "90.degree.". Measurement of the angle F is then achieved by separating the light wave output from the optic fiber into two light waves linearly polarized 90.degree. from one another. To achieve this, a 90.degree. separator-polarizer cube is used, which is an off-the-shelf item. Each of these two light waves is received on a respective photodiode, whose current consequently supplied is measured, respectively I1 and I2. The value of the angle F can then be deduced, by means of suitable electronic means, by applying the formula : EQU I1-I2)/(i1+I2)=sin 2F
A separator/polarizer cube however has the drawback of having a polarization ratio of 1 to 2% in intensity on each channel, which introduces a fairly penalizing signal distortion. Moreover, a separator cube of this kind is rather expensive, which increases the cost price of the sensor.
The second configuration set out in this article uses detection called "45.degree." which has the advantage of enabling a single optic fiber winding to be used to cover a wide range of current intensities to be measured. According to this second process, the light beam coming from the optic fiber after rotation F is separated into two components linearly polarized in two directions making an angle of 45.degree. between them, rather than 90.degree. as in the previous case. As a 45.degree. separator-polarizer cube cannot be found off-the-shelf, practical application of this process involves using a semi-reflecting blade and two WOLLASTON prisms each connected to a photodiode, which is even heavier and more costly than the previous method.
It is moreover shown that the intensities I1 and I2 collected in these two photodiodes are expressed by relations of the form: EQU I1=Io (1+cos 2F)/4 EQU I2=Io (1+sin 2F)/4
where Io is the intensity corresponding to the light source emitted by the above-mentioned laser diode.
Determining sin 2F and cos 2F requires the intensity Io corresponding to the source to be determined beforehand. In state-of-the-art assemblies, this value Io is determined by using a semi-reflecting blade, located upline from the optic fiber and a photodiode, which makes this type of installation even heavier.